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Hierarchical Gecko-Like Adhesives

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By Christian Greiner, Eduard Arzt, and Aranzazu del Campo*

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[*] Dr. A. del Campo, Prof. E. ArztLeibniz Institute for New Materials, Campus D2 266123 Saarbrucken (Germany)E-mail: delcampo@mf.mpg.de

Dr. C. GreinerUniversitat Stuttgart, INAMHeisenbergstrasse 3, 70569 Stuttgart (Germany)

DOI: 10.1002/adma.200801548

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Gecko feet constitute one of the most fascinating examples ofstrong but reversible adhesives. They are covered by millions offine hairs (setae) organized in a hierarchical arrangementspanning the millimeter to nanometer range.[1,2] It is nowwell-accepted that it is this special topography that allows thegecko to attach firmly to almost any surface. Inspired by thiscapability, scientists worldwide have tried to develop gecko-inspired surfaces.[3] However, the natural system remains so farunmatched. Recent work has experimentally demonstrated thatsurface-design parameters like pillar size, shape, aspect ratio, ortilt angle need to be considered when fabricating gecko-inspiredsystems.[4–9] Up to now, the influence of the hierarchicalarrangement of the fibrils has not been studied experimentally,although recent theoretical models predict the importance ofhierarchy in adhesion, especially to nonplanar surfaces.[10–16] Inthis contribution we present the first experimental investigationof the role of hierarchy in structured adhesive surfaces.Polydimethylsiloxane (PDMS) was structured with micropillarsusing a combination of two-step photolithography with SU-8resist and subsequent soft lithography. Two-level structuredsurfaces with pillars of different dimensions were obtained, andtheir adhesion properties were characterized in comparison withone-level structures. Contrary to expectations, we did not find animprovement with the present hierarchical structures whencompared with single-level pillars. We conclude that hierarchy isnot a relevant design parameter in adhesion of structuredsurfaces to planar substrates, but may be relevant in the case ofstructured surfaces composed of stiffer materials, or adhesion torough substrates, where adaptability of the adhesive structure isrequired.

Hierarchically structured PDMS surfaces were obtained by softmolding Sylgard 184 on photolithographic templates containinghierarchical holes. These were obtained in a double layer-by-layercoating-and-exposure step using SU-8 resist (type 5, 10, 20, and50, depending on the required resist thickness, Fig. 1). A singledevelopment step after the second exposure was sufficient todissolve the nonexposed regions from both layers. Homogeneousfields of 0.8 cm�0.8 cm could be patterned with holes of differentdimensions, and were subsequently used for soft molding. ThePDMS replicas were characterized by optical microscopy andscanning electron microscopy (SEM) to test the homogeneity ofthe film, and with white-light interferometry to accurately check

their dimensions and integrity (Fig. 2). The base pillars had adiameter of 50mm and a height of 200mm (aspect ratio l¼ 4),and the top pillars had a diameter of 5mm and heights rangingfrom 2.5 to 10mm (l from 0.5 to 2). Other radii for the top pillarswere also successfully fabricated (data not shown here). Thepillars were found to be nonplanar on the top, but had a finitecurvature (See Fig. 2A Supporting Information). This aspect isimportant, since the geometry at the contact point is relevant forthe final adhesion performance.[17,18] We assume that thiscurvature was also present in the lithographic template, and maybe a consequence of incomplete development. Pattern develop-ment involves diffusion of developer molecules into the non-cross-linked SU-8 regions and diffusion of solvated polymerchains into the developer solution. In the case of high-aspect-ratiofeatures, such as narrow and deep holes, completion of thisprocess requires long developing times, which in turn couldcause dramatic damage of fine features. Our developingconditions were optimized to yield well-defined holes, but mayhave produced slightly rounded bottom edges. Changes in theirradiation and baking times or in the sonication duringdevelopment did not improve the results.

Adhesion measurements of the hierarchical patterns wereperformed using our established technique.[5] A sphericalindenter with a radius much larger than the pillar dimensionsis placed in contact with the specimen under a steadily increasingapplied force (preload, Pp). Both the applied force and theindenter displacement are recorded. During retraction, the finaldetachment event gives the value of the pull-off force (Pc), whichis a measure of the adhesion performance. These data can betranslated into a pull-off strength through division by the(calculated) apparent contact area. Measurements were per-formed at different preloads, as this parameter has been shown toinfluence the Pc of structured surfaces.[5] Control experiments onsingle-level pillars with the same dimensions as the second-levelpillars with an aspect ratio of one were performed for comparison.

Figure 3 shows the adhesion force as a function of compressivepreload for two- and single-level pillars. It is immediately

Figure 1. Schematic of the processing steps for the fabrication of hierarchi-cal PDMS pillars through two-step photolithography and soft molding.

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Figure 3. Effect of top-pillar aspect ratio on adhesion, plotted as pull-offforce versus preload. The bottom pillars had a radius of 25mm and anaspect ratio of 4 in all experiments. The results for single-level pillars withthe shape of punches with rounded edges and an aspect ratio of 1 havebeen added (scale on right ordinate).

Figure 2. a) SEM image showing an array of hierarchical pillars fabricatedby soft molding Sylgard 184 on SU-8 photolithographic templates. Thebase-pillars have a radius of 25mm and a height of 200mm. The top pillarshave a radius of 5mm and an aspect ratio of 1. The right insert shows aclose-up of the hierarchical structure, the left a water droplet resting on thestructure (contact angle 1608). The arrows indicate pillar sections. b) SEMimage of single-level pillars having the same dimensions as the top ones ina). Note the difference in pillar packing density between both specimens.Samples were coated with 10 nm Au/Pd before imaging.

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apparent that the two-level pillars adhere about a factor of ten lessthan the single-level structures with rounded edges (note thedifferent scale on the ordinates). The same is true for the pull-offstrength (Fig. 2A of the Supporting Information). This result issurprising, as theoretical studies have predicted a beneficial effectof hierarchy in adhesion.[10–13,15,19] However, some issues need tobe pointed out. First, the second-level pillars in the hierarchicalstructure have a much lower packing density (ca. 5.15%) than thesingle-level structures (22.67%) – compare Figure 1a and b –,which is expected to reduce adhesion by a factor of 4.4. A secondobservation is the partial misalignment of the top pillar and thebase-pillars (as a consequence of the fabrication method), whicheffectively ‘‘sectioned’’ some of the top pillars (see arrows inFig. 1a). It is difficult to quantify the effect of themisalignment onadhesion, but we believe that it contributes by another factor of 2.

Figure 3 also compares Pc of the hierarchical structures withsingle-level pillars having an aspect ratio of 1 and showing theshape of a punch with rounded edges. As we previously foundwith single-level structures,[5] the adhesion performanceincreases with increasing aspect ratio. This effect has beenattributed to a higher elastic-energy dissipation at pull-off,[20–23] inanalogy with the mechanism of crack propagation in rubberymaterials.[22] Assuming full dissipation, the effective work ofadhesion is increased from g to geff:

[5]

geff ¼ g þ f s�2r

E� l (1)

In this equation, f represents the pillar packing density, r is the2

pillar radius, and E*¼E/(1� n ) is the reduced Young’s modulus,

with a Poisson ratio of n¼ 0.5, and s* is the interfacial strength.Taking s*¼ 1 MPa,[20] E*¼ 1.43 MPa, g ¼ 0.068 J m�2, andf¼ 0.2267 from reported studies with the same material,[5] weobtain geff¼ 0.464 J m�2 and 1.653 J m�2 for l¼ 0.5 and 2. In theclassic Johnson, Kendall and Roberts (JKR) solution for thecontact of a sphere with a flat surface, Pc is expected to scalelinearly with the work of adhesion.[24] This results in a theoretical3.6-fold increase in Pc, when raising l from 0.5 to 2.Experimentally, we found an increase of a factor 3.2 (from0.23 to 0.74 mN).

At this point, it is of interest to compare our results with thescarce data on hierarchical structures reported in the literature.Hierarchical arrays of fibrils have been fabricated by fillingstacked alumina membranes with different pore sizes.[25] Fibrilsof 10mm diameter and 70mm length were decorated withnanofibers with 60 nm diameter and aspect ratios of up to 100.However, fiber collapse and clumping, which cancels anybeneficial effects that the hierarchical structure[17] may have,prevented adhesion measurements. In a different approach, SiO2

platforms microfabricated on a flat silicon wafer were coated withirregular fibrils with �200 nm diameter and �4mm length.[26]

The adhesion strength was found to be sc¼ 0.028 kPa for thehierarchical structure, a factor of 4 to 5 higher than a fibril-coatedflat silicon wafer without the platforms. Our two-level hierarchicalstructure shows adhesion strength higher by factor of 75 (sc forl¼ 2 at Pp¼ 1 mN is about 2.15 kPa).

Some theoretical works have investigated the effect ofhierarchy on the adhesion of fibrillar structures. Based on afractal-like hierarchical hair model, an increased work of

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adhesion with increasing structural levels of hierarchy waspredicted by Yao et al.[10–12] These predictions contrast with ourexperimental results, but the authors pointed out that thenanometer length scale, as with the gecko adhesive system, playsa crucial role. Our structures are microsized, and therefore theelastic properties and geometry of the second-level pillars werealso predicted to strongly influence the traction-separationbehavior of the base pillar.[10] In fact, this could be neatlyinvestigated with our model system. We determined the effectiveYoung’s modulus of the structured surfaces by fitting thecompressive parts of the load-displacement curves with thestandard Hertzian expression:[27]

P ¼ 4

3E�

ffiffiffiffiffiffiffiffiRd3

p(2)

where P is the applied compressive preload, R is the radius of the

indenting sphere, and d is the indentation depth.[4,5] A decrease in

the effective Young’s modulus by a factor between 3 and 7 was

experimentally found between our hierarchical and single-level

structures with different aspect ratios. Such a reduction can

explain the strong reduction in pull-off strength. At a given

preload, more (calculated) contact area with the indenting sphere

is produced, as the sample is more compliant and the sphere can

penetrate deeper into the material.Hierarchical structures were also modeled as arrays of springs

of different numbers, stiffnesses, and levels of hierarchy.[13,15,19]

Multilevel structures were predicted to produce higher adhesiveforces and energies than the single-level ones at a given preload.However, calculations considered adhesion against roughsubstrates, where the advantage of a hierarchical structure isobvious. Our results suggest that the hierarchy does not improveadhesion against flat substrates, even though the effectivestiffness decreases significantly upon addition of another levelof hierarchy.

Finally, we note in passing that the wettability of the surface isalso affected by hierarchy.Wemeasured a contact angle of 1088 onthe flat PDMS surface, and 1398 and 1608 on the single-level andtwo-levels pillars with radius 10mm (top pillars) and roundededges, respectively. Clearly, a hierarchical arrangement rendersthe surface almost superhydrophobic (see insert in Fig. 1b).

In summary, we have shown a novel approach to fabricatepatterns of hierarchical elastomeric micropillars. These struc-tured surfaces constitute a model system for investigating theparameter space relevant for gecko-like adhesives. For the firsttime, we have systematically tested and quantified the effect of ahierarchical arrangement of patterned pillars in the finaladhesion performance of a structured surface. As our structureswere not perfect due to incomplete alignment of the levels, theadhesion performance was lower than expected. Nevertheless,some generic conclusions could be drawn. In particular, we foundthat the adhesion of fibrillar structures does not benefit fromadditional levels of hierarchy when tested against planar surfaces.Further studies will be necessary to confirm the benefits ofhierarchy for adhesion of stiffer materials and for adhesion torough substrates, where adaptability of the adhesive structure isrequired. Experiments with pillars with smaller radii anddifferent tip-geometries are also in progress.

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Experimental

Materials and Equipment: The Silicon Wafers (100 orientation) wereprovided by Crystec (Berlin, Germany). SU-8 type 5, 10, 20, 50 and thedeveloper mr-Dev 600 were provided by Micro Resist Technology (Berlin,Germany). Hexadecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane was pur-chased from ABCR (Karlsruhe, Germany). Masks for lithography wereprovided by ML&C (Jena, Germany) in quartz with 0.8� 0.8 cm2 chrome-patterned fields. A mask aligner Karl Suss MJB4 (Garching, Germany) wasused for the irradiation step. Sylgard 184 was purchased fromDow Corning(MI, USA). The height and shape of the pillars were characterizedaccurately using white-light interferometry (ZYGOLOT 5000) and scanningelectron microscopy (LEO 1530VP). The static water contact angles weremeasured using a OCA 30 Contact Angle System using SCA 202 Software(DataPhysics Instruments GmbH, Filderstadt, Germany). The equipmentwas combined with a charge-coupled device (CCD) camera for imagecapturing.

Fabrication of Hierarchical SU-8 Hole Patterns by Two-StepLithography: In a first step, a pattern with 10mm structures was imagedon the SU-8 film. After a second coating step for a 200mm thick film,features with 50mmdiameter were imaged on the top of the 10mmpattern.Development was performed in a single step after the second irradiation.Detailed irradiation, baking, and development times used for eachgeometry can be found in the Supporting Information.

Fabrication of Single-Level Pillars with Rounded Edges: These structureswere fabricated by a double molding process using photolithographictemplates. Details have been published elsewhere.[18,28]

Silanization of the SU-8 Templates: Gas-phase silanization of theSU-8 structured wafers was performed with Heptadecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane in an evacuated desiccator for 30min.Afterwards, the wafers were baked for 30min at 95 8C under vacuum.Silanization with the perfluorosilane increased the contact angle of thesilicon wafer from 108 to 1138, and for the cured SU-8 from 738 to 1158.

Fabrication of Hierarchical PDMS-Pillar Patterns by Soft Molding: Amixture with 10:1 ratio of Sylgard 184 prepolymer and crosslinker wasdegassed and poured on a silanized SU-8 patterned wafer. Curing wasperformed for 14 h at 65 8C in light vacuum (�6� 104 Pa). The totalthickness of the elastomer samples was controlled with a Teflon ring ofdefined height, which surrounded the wafer. Standard samples had athickness of 5mm.

Adhesion Measurements: Measurements were performed using home-built equipment. This apparatus and the measurement principle have beenreported previously.[4,5,29]

Acknowledgements

This project has been funded by the Volkswagen Foundation. SupportingInformation is available online from Wiley InterScience or from the author.This article is part of a Special Issue on Biomaterials.

Received: June 5, 2008

Revised: July 21, 2008

Published online: November 14, 2008

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